Could having a heart bypass have made fluid accumulate in my father's lungs a few days later?

Possibly. The bypass should help heart function. However it is a very stressful surgery. If your father's heart already had poor function and unrecoverable tissue damage, he is at high risk of congestive heart failure. A lot of fluids are needed and how well the heart handles them can be unpredictable. I am sure the surgical team looked for evidence of graft malfunction or clotting, which may need surgery.
Yes. This is not an uncommon complication. Fluid in the lung tissue itself could mean congestive heart failure and that is a treatable situation. Most commonly, the fluid accumulates in the pleural space, either from residual bleeding or an inflammatory reaction to the surgery, if it doesn't resolve on its own it can be drained, .